This invention relates to a sprue gated multi-cavity injection molding system and more particularly to an integral electrically heated manifold and a method of manufacturing the same.
In the past, injection molding systems have been well known which have manifold plates with several arms through which melt passages branch outwardly from a common inlet to convey melt to a number of cavities. An example of this type of structure is shown in U.S. Pat. No. 4,013,393 entitled "Valve-Gated Injection Molding Mechanism" which issued Mar. 22, 1977 to Gellert. It is, of course necessary that the temperature of the hot melt be maintained within a certain range as it flows through the manifold plate. If the temperature of the melt becomes too high it will partially decompose, and if it becomes too low it will become too viscous. A number of systems are accordingly known for providing heat to the manifold plates and recent improvements in this regard are shown in Gellert Canadian patent application Nos. 393,671 and 400,058. Application No. 393,671 which was filed Jan. 6, 1982 and entitled "Injection Molding Manifold Member and Method of Manufacture" discloses casting copper around a heating element in a channel in the manifold plate formed of H13 steel. Application No. 400,058 which was filed Mar. 13, 1982 entitled "Heater Installation in Molding Members" describes a method of integrally casting a cartridge heater into an H13 steel manifold plate.
While both of these structures provide improvements in heat transfer and even temperature control, the constant expansion of the application of injection molding to more and more difficult to mold materials has made abrasion and corrosion resistance as well as temperature control even more critical. Furthermore, a deterioration in the quality of available molding materials has resulted in existing systems being more susceptible to corrosion which, on occasion, results in their no longer being acceptable for existing applications. Therefore, for certain corrosive or abrasive materials, it is necessary that the melt passage extend completely along its length through a corrosion and abrasion resistant material such as stainless steel. However, this must be combined with the provision of very reliable temperature control to maintain the melt within the narrow critical range.
More recently, it has become desirable to provide an improved manifold for a relatively straight forward sprue gated multi-cavity system which has a small number of heated nozzles or sprue bushings of the type disclosed in U.S. Pat. No. 4,238,671 entitled "Sprue Bushing with Cast In Heater Element" which issued Dec. 9, 1980 to Gellert. Improved nozzles of this type are also shown in Gellert U.S. C-I-P application Ser. No. 234,641 which was filed on Feb. 17, 1981, now U.S. Pat. No. 4,355,460, issued Oct. 26, 1982 entitled "Sprue Bushing and Method of Manufacture" and Gellert U.S. patent application Ser. No. 285,260 which was filed July 20, 1981, now U.S. Pat. No. 4,403,405, issued Sept. 13, 1983 entitled "Sprue Bushing Connector Assembly and Method". In addition to having this improved combination of characteristics, it is desirable that the manifold for such a system be relatively economical to manufacture. Furthermore, it is a considerable economical advantage to overcome the disadvantage of these previous manifold plates that the melt passages are relatively difficult to clean out if the system malfunctions and the melt decomposes or solidifies in them.